Platinum group metals such as platinum and palladium are employed in industrial catalysts, catalysts for purifying automotive exhaust gas, and various electrical products. Since such platinum group metals are expensive and useful as resources, they have been recovered and recycled after the use. Furthermore, recently, in consideration of securing the resources, importance of recovery and recycling has been further increasing.
For recovering the platinum group metals, various methods such as a sedimentation separation method (Patent document 1), an ion exchange method (Patent documents 2 and 3), an electrolytic precipitation method (Patent document 4), and a solvent extraction method have been proposed and exercised. Among these methods, the solvent extraction method is widely adopted for economical property and operability.
In the solvent extraction method, sulfur-containing organic compounds and organic phosphorous compounds have been employed. As the sulfur-containing organic compounds, dialkyl sulfide and dialkylsulfine oxide are utilized, and, as the organic phosphorous compound, trialkyl phosphonate, trialkyl phosphate, trialkylphosphine oxide and trialkylphosphine sulfide have been known, and there has been known a method of utilizing dialkyl sulfide (DAS) or tributylphosphoric acid (TBP) (Patent document 5). According to this method, palladium is extracted with DAS, and platinum is recovered by TBP. Since osmium and ruthenium are also extracted in DAS, there are required, in order to prevent such a contamination, operations of dissolving in aqua regia, oxidizing osmium and ruthenium into tetroxide, and subsequent heating for elimination by evaporation (Patent document 6).
However, this method involves problems such as requiring a large amount of oxidant. Because of this fact, it is considered effective to utilize DAS of a four times molar amount or larger with respect to the platinum group metals (Patent document 7). Among DAS, dihexyl sulfide (DHS) is generally utilized, but it is pointed out that there is a difficulty in the extracting speed.
Patent document 1: JP-A-10-102156
Patent document 2: JP-A-3-22402
Patent document 3: JP-A-7-310129
Patent document 4: JP-A-8-158088
Patent document 5: JP-A-63-14824
Patent document 6: JP-B-1-30896
Patent document 7: JP-A-9-279264